Recently an effective block approach has been proposed to address the experimentally observed growth rates of fatigue cracks at critical locations on F/A-18 airframes. In this approach, each program of spectrum load is treated as an equivalent constant amplitude cycle, and the baseline crack growth rate data are obtained using a similar spectrum load of interest. A procedure was devised to allow the use of the model parameters obtained under one load spectrum to predict the crack growth under a different load spectrum. In this study, we critically evaluate the capability of the effective block approach, using data obtained for the F-111 and P-3C coupon test programs, to gauge its general applicability to other aircraft operated by the Royal Australian Air Force. The data used in the evaluation encompasses different load spectra, different materials and different crack configurations. This investigation has found that the effective block approach was able to model fatigue crack growth in 2024-T851 aluminium under the F-111 flight spectra examined, but it could not produce an acceptable estimation of the total crack growth life for the P-3C spectra studied. It was, however, able to produce reasonable predictions of fatigue crack growth in a chosen interval of crack length. This report provides an independent evaluation and guidance for the application of the effective block approach.